Synthesis and Color Evaluation of Tb⁴⁺-Doped Na₂ZrO₃ for Inorganic Yellow Pigments

Abstract

Tb⁴⁺-doped sodium zirconate samples, Na₂Zr_1−xTb_xO₃, were synthesized as novel environmentally friendly inorganic yellow pigments by a conventional solid-state reaction method. Their crystal structures, optical properties, and colors were characterized. A single-phase form was obtained for the samples in the x range of x ≤ 0.18, while impurity phases were detected for the sample with x = 0.20. All samples showed strong optical absorption in the blue light region, due to the charge transfer transition between O²⁻ and Tb⁴⁺. As a result, the sample color became yellow, which is the complementary color of blue, and the color became more vivid with increasing Tb⁴⁺ content in the single-phase region. Among the samples, Na₂Zr₀.₈₂Tb₀.₁₈O₃ was the optimal composition, with the highest yellowness (b* = +67.2) and pure yellow hue (h° = 90.1). Although the b* value was lower than commercial yellow pigments such as BiVO₄ and ZrSiO₄:Pr, this sample had a purer yellow hue. Since Na₂Zr_0.82Tb_0.18O₃ is composed of non-toxic elements, it could be a new environmentally friendly inorganic yellow pigment.

1. Introduction

Inorganic pigments have been widely applied to products such as inks, paints, ceramics, and so on. Particularly, yellow pigments are used in paints for warning signboards and traffic road markers owing to their high visibility [1]. Chrome yellow (PbCrO₄), cadmium yellow (CdS), and nickel–titanium yellow (TiO₂-NiO-Sb₂O₃) pigments have been used as industrial yellow pigments. However, these pigments contain elements (e.g., Pb, Cr, Cd, and Sb) that are toxic to humans and the environment. The use of these existing pigments has so far been prohibited or restricted, and there has been a need to develop novel yellow pigments. In light of this situation, a number of studies have been reported by several researchers [2,3,4,5,6,7,8,9,10,11,12,13]. However, there are few high-performance inorganic yellow pigments that are environmentally friendly, demonstrate sufficient chemical/thermal stability, and comparable to the conventional products. In addition, oxide materials that are highly stable and can be synthesized in processes that do not use toxic gases are desired to provide more sustainable and environmentally friendly materials.

In view of this situation, we focused on Tb⁴⁺ as a yellow coloring source and proposed a new concept in the development of yellow pigments. Some compounds such as A(Sn, Tb)O₃ (A = Sr, Ba) [14,15,16,17], A₂(B, Tb)O₄ (A = Sr, Ba; B = Zr, Sn, Ce) [18,19,20], and Bi₃(Y, Tb)O_6+δ [21] have been proposed as environmentally friendly yellow pigments. These compounds absorb visible light in the blue light region due to the charge transfer (CT) transition between O²⁻ and Tb⁴⁺ and exhibit a yellow color. In most cases of these materials, the Tb⁴⁺ ions are introduced into the octahedral [MO₆] (M = metal) sites. Therefore, when Tb⁴⁺ ions are doped into [MO₆] octahedra, a yellow color will be obtained.

In this study, we selected sodium zirconate, Na₂ZrO₃, as a host material. This compound is composed of only non-toxic and cost-effective elements. A Na₂ZrO₃ structure includes octahedral [ZrO₆] sites [22,23,24,25] and the valence of Zr in this structure is tetravalent. The valence of Zr⁴⁺ is equivalent to Tb⁴⁺ and the ionic radius of Zr⁴⁺ (0.072 nm [26]) is close to that of Tb⁴⁺ (0.076 nm [26]) in size. For these reasons, we thought Tb⁴⁺ ions could be introduced into the octahedral Zr⁴⁺ site to exhibit a yellow color. Therefore, in this study, Tb⁴⁺-doped Na₂ZrO₃ samples were synthesized, and their optical and color properties were characterized as environmentally friendly inorganic yellow pigments. Considering the viewpoint of practical use, the sample color was compared with commercial inorganic yellow pigments, and the chemical stability was also evaluated.

2. Materials and Methods

2.1. Synthesis

The Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) samples were synthesized via a solid-state reaction method. Stoichiometric amounts of ZrO₂ (FUJIFILM Wako Pure Chemical, 98.0%, Fujifilm, Tokyo, Japan) and Tb₄O₇ (FUJIFILM Wako Pure Chemical, 99.9%) and 1.6 times the stoichiometric amounts of Na₂CO₃ (Kanto Chemical, Tokyo, Japan, 99.5%) were mixed in an agate mortar for 30 min to obtain 2 g of the final product. The mixtures were calcined in an alumina crucible at 1200 °C for 5 h in air. Finally, the samples were ground in an agate mortar before characterization.

2.2. Characterization

The crystal phase and structure were identified by X-ray powder diffraction (XRD; Ultima IV, Rigaku, Tokyo, Japan). The XRD patterns were captured with Cu-Kα radiation operating at 40 kV and 40 mA. The data were collected by scanning over the 2θ range of 20–80°. The sampling width was 0.02° and the scan speed was 6° min⁻¹. The lattice volumes were calculated from the XRD peak angles refined by α-Al₂O₃ as a standard and using the CellCalc Ver. 2.20 software. The morphologies were observed by using field-emission-type scanning electron microscopy (FE-SEM; JEOL, Tokyo, Japan, JSM-6701F).

To investigate optical properties of the powder samples, the optical reflectance spectra were recorded on an ultraviolet–visible–near-infrared (UV–Vis-NIR) spectrometer (JASCO, Tokyo, Japan, V-770 with an integrating sphere attachment) using a standard white plate as a reference. The step width was 1 nm and the scan rate was 1000 nm min⁻¹. The color properties of the powder samples were evaluated in terms of the Commission Internationale de l’Éclairage (CIE) L*a*b*Ch° system, using a colorimeter (Konica-Minolta, Tokyo, Japan, CR-300). The L* parameter indicates the brightness or darkness in a neutral grayscale. The a* and b* values represent the red–green and yellow–blue axes, respectively. The chroma parameter (C) denotes the color saturation and is calculated with the formula, C = [(a*)² + (b*)²]^1/2. The hue angle (h°) ranges from 0 to 360° and is estimated according to the following formula: h° = arctan(b*/a*). The standard deviations of all values for the L*a*b*Ch° color coordinate data were less than 0.1.

3. Results and Discussion

3.1. X-ray Powder Diffraction (XRD) and Scanning Electron Microscopy (SEM)

Figure 1 shows the XRD patterns of the Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) samples. In the x range from 0 to 0.18, the Na₂ZrO₃ phase was obtained in a single-phase form. For x = 0.20, on the other hand, the target phase was observed as the primary phase, but additional impurity peaks indexed to ZrO₂ and Tb₁₁O₂₀ were also detected.

Figure 1. XRD patterns of the Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) samples.

The crystal structure of Na₂ZrO₃ belongs to a monoclinic system with a symmetry of the C2/c (no. 15) space group. In the Na₂ZrO₃ structure, both Na⁺ and Zr⁴⁺ coordination numbers are six and there are [NaO₆] and [ZrO₆] octahedra in the lattice [22]. The composition dependence of the lattice volume of the samples calculated from each XRD pattern is shown in Figure 2. The lattice volume was found to increase linearly with increasing Tb⁴⁺ content in the x ≤ 0.18 range by introducing larger Tb⁴⁺ (ionic radius: 0.076 nm [26]) ions into the Zr⁴⁺ (ionic radius: 0.072 nm [26]) sites. The cell volume at x = 0.20 was almost equal to that at x = 0.18. Therefore, solid solutions of Na₂Zr_1−xTb_xO₃ were successfully synthesized in the region of x = 0 to 0.18.

Figure 2. Compositional dependence of the lattice volume for the Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) samples.

Figure 3 shows the SEM images of the Na₂Zr_1−xTb_xO₃ (x = 0 and 0.18) samples obtained in a single-phase form. The size of the primary particles in both samples was approximately 5 µm. The primary particles were thermochemically fused to form coarse secondary particles, due to the high calcination temperature of 1200 °C.

Figure 3. SEM images of the Na₂Zr_1−xTb_xO₃ (x = 0 and 0.18) samples; (a) x = 0, (b) x = 0.18.

3.2. Ultraviolet-Visible (UV-Vis) Reflectance Spectra

The UV–Vis reflectance spectra of Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) are depicted in Figure 4. The host material exhibited high reflectance in the visible region and was white in color. Except for the Na₂Zr_0.80Tb_0.20O₃ (x = 0.20) sample, the Tb⁴⁺-doped samples strongly absorbed the blue light at wavelengths between 435 and 480 nm, and this optical absorption was attributed to the charge transfer (CT) transition between O²⁻ and Tb⁴⁺ [17,19]. Since blue is the complementary color of yellow, these samples are yellow. On the other hand, the sample with x = 0.20 absorbed visible light in the longer wavelength region. The impurity phase, Tb₁₁O₂₀, detected in Figure 1 contains both Tb³⁺ and Tb⁴⁺. This compound absorbs visible light through charge transfer transitions between metal ions with different valence [27]. On the other hand, the other impurity, ZrO₂, is white in color and generally shows no optical absorption in the visible light region. Therefore, the additional optical absorption observed in the sample with x = 0.20 was attributed to charge transfer transitions between Tb³⁺ and Tb⁴⁺.

Figure 4. UV–Vis reflectance spectra for the Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) samples.

As can be seen from Figure 4, the optical absorption intensity corresponding to the O²⁻–Tb⁴⁺ CT transition increased with increasing Tb⁴⁺ concentration. Among the samples with x ranging from 0 to 0.18 (solubility limit), the Na₂Zr_0.82Tb_0.18O₃ (x = 0.18) sample showed the highest absorption intensity.

3.3. Color Properties

The L*a*b*Ch° color parameters of the Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) samples and commercially available yellow BiVO₄ (Dainichiseika Color & Chemicals Mfg., Tokyo, Japan), PbCrO₄ (NIC, Osaka, Japan) and ZrSiO₄:Pr (Kawamura Chemical, Yokkaichishi, Japan) pigments are summarized in Table 1. The photographs are also displayed in Figure 5. The host Na₂ZrO₃ sample had a high L* value (brightness), and both a* (redness) and b* (yellowness) values were almost equal to zero. This sample exhibited a white color.

Table 1. Color coordinate data of the Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) samples and the commercially available yellow BiVO₄, PbCrO₄, and ZrSiO₄:Pr pigments.

Sample	L*	a*	b*	C	h°
Na2ZrO3	98.0	−0.14	+0.62	0.64	103
Na2Zr0.95Tb0.05O3	91.5	−6.95	+50.9	51.4	97.8
Na2Zr0.90Tb0.10O3	86.4	−3.70	+61.5	61.6	93.4
Na2Zr0.85Tb0.15O3	82.6	−0.62	+66.1	66.1	90.5
Na2Zr0.82Tb0.18O3	82.4	−0.17	+67.2	67.2	90.1
Na2Zr0.80Tb0.20O3	66.2	−1.26	+46.5	46.5	91.6
BiVO4	93.3	−15.7	+80.3	81.8	101
PbCrO4	89.9	+1.12	+96.5	96.5	89.3
ZrSiO4:Pr	83.5	−3.28	+70.3	70.4	92.7

Figure 5. Photographs of the Na₂Zr_1−xTb_xO₃ (0 ≤ x ≤ 0.20) samples and the commercial BiVO₄, PbCrO₄, and ZrSiO₄ pigments.

When Zr⁴⁺ was partially replaced by Tb⁴⁺, the b* value increased dramatically due to the strong absorption of blue light, as shown in Figure 4. In addition, the h° parameter (hue angle) fell into the yellow region of 70 ≤ h° ≤ 105. As a result, the Tb⁴⁺-doped samples turned yellow. Furthermore, the color of the samples changed from pale to bright yellow as the absorption intensity in the blue light region increased with increasing Tb⁴⁺ content. Among the samples synthesized in this study, the highest b* and C (chroma) values were obtained for the Na₂Zr_0.82Tb_0.18O₃ sample, which exhibited the brightest yellow color.

The color coordinate data of the Na₂Zr_0.82Tb_0.18O₃ sample in this study were compared with those of the commercially available yellow pigments, as shown in Table 1 and Figure 5. The yellowness (b*) of this sample was slightly lower than that of the environmentally friendly ZrSiO₄:Pr pigment. However, the h° value of this sample was closer to 90° (pure yellow) than the commercially available BiVO₄ and ZrSiO₄:Pr pigments. Because the standard deviations of all values for the L*a*b*Ch° color coordinate data were less than 0.1, there was a significance difference between the Na₂Zr_0.82Tb_0.18O₃ pigment and the commercial ones, and we concluded this pigment had a purer yellow color than them.

3.4. Chemical Stability Test

The chemical stability of the Na₂Zr_0.82Tb_0.18O₃ sample was evaluated using the powder sample. The powder samples were soaked in deionized water, 4% CH₃COOH, and 4% NH₄HCO₃ aqueous solutions. The samples were left at room temperature for 5 h (for deionized water) and 2 h (for acid and base) and then washed with deionized water and ethanol. The samples were then dried at room temperature. The color coordinate data of the samples after the chemical stability tests are summarized in Table 2, and their photographs are also displayed in Figure 6. Unfortunately, the chemical stability of the Na₂Zr₀.₈₂Tb₀.₁₈O₃ sample was insufficient, and each treatment resulted in degradation of the color tone. Therefore, surface coating with a stable compound such as silica is considered necessary for the application of this sample to inorganic pigments. Silica is considered suitable as a coating material because it is a cost-effective, stable, and green compound. If the pigment in this study was coated with silica, we expect it would be a more environmentally friendly yellow pigment than uncoated pigments.

Table 2. Color coordinate data of the Na₂Zr_0.82Tb_0.18O₃ sample before and after the chemical stability test.

Treatment	L*	a*	b*	C	h°
As synthesized	82.4	−0.17	+67.2	67.2	90.1
Water	62.1	+17.2	+60.4	62.8	74.1
4% CH3COOH	63.1	+15.8	+65.6	67.5	76.5
4% NH4HCO3	63.5	+16.1	+61.1	63.2	75.2

Figure 6. Photographs of the Na₂Zr₀.₈₂Tb₀.₁₈O₃ sample before and after the chemical stability test.

4. Conclusions

Tb⁴⁺-doped sodium zirconate samples, Na₂Zr_1−xTb_xO₃, were synthesized as environmentally benign inorganic yellow pigments by a solid-state reaction method. In the x range from 0 to 0.18, the samples were obtained in a single-phase form and the lattice volume increased linearly. These results indicate that the solid solutions were successfully obtained in this x region. The Tb⁴⁺-doped samples exhibited optical absorption in the blue light region due to the charge transfer transition between O²⁻ and Tb⁴⁺. The intensity of this optical absorption increased with increasing Tb⁴⁺ content. As a result, the color of the samples changed from pale to bright yellow. Among the samples synthesized, the Na₂Zr_0.82Tb_0.18O₃ sample showed the highest yellowness. The yellowness of this sample was inferior to those of commercially available yellow pigments. However, the yellow color purity of this sample was better than those of the existing environmentally friendly types. In addition, no toxic solvents or gases are required or generated in the synthesis of this pigment. Therefore, although the chemical stability of the present sample is not sufficient, it may become a new environmentally friendly inorganic yellow pigment by coating with a stable compound.